Great tutorial! Another way you can do the boat rudder bindings, is to bind it to the STEERING axis instead of roll. That way it will also be independent of how you have bound your Yaw and Roll axes, and it shouldn't need explaining. The Steering axis is mainly used for car steering, so it should be a good fit for boat steering as well.
Hey, the only way I know of is to delete your old root part, the game will ask you to select another. Make sure you make a backup copy of your craft. It may also be possible to change the root part via the craft file in notepad. Haven't tested that tho.
